Lecture week 13 (11-16) - Notes Community Ecology - Succession.docx

Biol 114 Fall 2023

Lecture Week 13 (11/16)

Community Ecology: Succession

1. Introduction
   1. As we know from our classes on Evolution, Mendelian Genetics, Population Genetics, Population Ecology, and Exploitative and Mutualistic Interactions the biotic elements of our environment are continually changing. Across these lectures, we have changed scale from what happens to the individual (Selection, Mendelian Inheritance), to the population (Evolution, Population Genetics, Population Ecology), and to between species (Competition, Exploitation and Mutualism). We are now going to change scale again and consider events at the Community level.
   2. Community Ecology may be defined as how species interact within a community. And there is some truth that the interspecific interactions we have covered thus far represent some understanding of community ecology. However, in application, community ecologists are as concerned with the community as a whole as they are with the individual interacting parts. That is, community ecologists are interested in the species diversity within a community and how that diversity changes over time, and what causes those changes (or how might we predict those changes). Community ecology is the study of how a community, as a whole, functions and changes over time.
   3. Before we start talking in earnest about community ecology, we must first determine what is meant by “species diversity”. Species Diversity refers to the number and relative abundance of species in an area. Whereas Species Richness refers to the number of different species in an area. Typically, people are really referring to species richness when they say “species diversity”. As an example, consider the two forests of equal size, and numbers of trees within those forests, below:

Forest #1 Forest #2

Species Number Species Number

Eastern Hemlock 15 Eastern Hemlock 33

Sugar Maple 59 Sugar Maple 20

American Beech 37 American Beech 58

Total 111 111

Both forests have 3 species in them. Therefore, they have the same species richness. Additionally, they have the same total number of individual trees (111). However, the relative numbers of each species is different between them (e.g., over 50% of Forest #1 is made of sugar maples, where sugar maples represent fewer than 20% of the trees in forest #2). Therefore, the two forests have a different species diversity.

When people say that we need to “maintain species diversity” in a particular ecosystem, or a “lack of diversity” is the downfall of an ecosystem, they are really talking about species richness.

Is this too picky? Maybe, but consider this. In order to take care of the planet, we all need to know what others are talking about. We cannot afford to get lost in conceptual confusion. The potential costs are too great.

1. Community Change
   1. Succession is the gradual change in an area over time. That is, it is the changing of communities, in species diversity and richness. When change occurs, different species may be present in an area, where they were not before, and relative abundance of existing species may be different as well.
      1. Successional study may come in the form of Vegetative Succession, Animal Succession, or Geologic Succession. Within these categories, you may also consider things such as Stream Succession, Old Field Succession, and Aquatic Plant Succession, to name a few. Literally, any change over time in an area can be a study in succession.
      2. When people talk of succession, they generally mean vegetative succession for a couple of reasons. First, it’s one of the more well-studied forms of succession, and we are at the point where we can make generalizations about succession based on data from vegetative succession studies. Second, vegetative succession tends to be more predictable than other forms of succession. Third, with changes in vegetation over time, you typically get predictable changes in other areas as well. For example, with the change from old field to shrubs to pioneer trees, you tend to get animals (vertebrates and invertebrates) that favor each vegetation stage only, and the diversity and richness of animal species changes with the vegetation over time.
   2. Types of Succession:
      1. Primary Succession: results from the creation of new substrates, such as by volcanoes (deposition of volcanic ash), shifting sand dunes, and glaciers. No original soil remains and new soil must be created again by pioneer plants and their decomposition.
      2. Secondary Succession: arises when the community is diminished but the soil remains intact, such as following wind, fire, overgrazing, logging and hurricanes, to name a few.
         1. Primary succession starts with bare rock (or rock pieces) continues through lichens and mosses then to herbaceous species.
         2. Secondary succession starts with exposed soil and moves into herbaceous species.
         3. Example: Glacier Bay, Alaska
      3. Traditionally, vegetative succession follows a specific pattern of change. For example: bare soil to herbs to shrubs to pioneer trees to intermediate trees to climax forest.
      4. A local community is often defined by what is or will be the species number and composition of the late successional stage (climax) at that location. For example, the area around the Binghamton University campus is considered a sugar maple, hemlock forest even though there are many areas on and around campus that do not include maples or hemlocks. The assumption is that if the natural areas here were left to change on their own, they would end up being dominated by maples and hemlocks.
      5. Be careful! It is incorrect to think of succession as necessarily leading to a local climax forest, and it is incorrect to assume that succession must follow the progression described in #3 above. It is entirely likely that a local habitat will never reach the climax stage typified in the area, especially if repeated disturbances interrupt the flow of succession. Additionally, successional stages may be skipped or repeated, depending on local circumstances. The pattern presented in #3 above is considered an idealized model of change only.
      6. Therefore, you can say that the pathway, or progression, of succession in any area is influenced by two general factors: Randomness and the History of the area. Over time, events may randomly occur that destroy the current suite of community members (e.g., say a fire burns a forest down). As a result, succession doesn’t happen in the predicted way. Additionally, certain stages may be skipped because there may not be source populations of the species of that stage to help colonize the area (a reflection of the history of the area).
   3. 3 mechanisms describing succession: How does succession take place?
      1. Facilitation: One species makes it easier for subsequent species to live in the same habitat. For example herbaceous species like legumes help to fix nitrogen and make it available to other species. Shrubs and trees would not be able to exist in areas without this preparation of the soil.
      2. Tolerance: Some species show greater tolerance of extremes in variation in abiotic factors involving light, temperature, wind and dryness. Later succession stages show greater tolerance of a variety of abiotic factors, and are thus better competitors for resources. For example, shrubs are more tolerant of shade than herbs. Pioneer trees are more tolerant of shade than shrubs are, etc.
      3. Inhibition: early succession plants are inhibited by later succession plants in a mature community (late succession species are more tolerant of subdued light (as in the shade example in #2 above)). Also, some herbaceous species inhibit the growth of shrubs by outcompeting young shrubs for resources.
      4. There has been some debate about which of the 3 mechanisms is the primary driver of succession. However, generally, all three play a part in successional change in most habitats.
2. Stability in Communities

Stability in a community refers to the ability of the community to resist abrupt changes in species diversity and richness. Generally, a stable community is one that is thought to be healthy. Abrupt changes here include the addition of an invasive, foreign species into, or the extinction of a species from, the community.

• 1. Co-evolutionary relationships between species tend to add stability, because each buffers the other from the adverse effect of many biotic and abiotic factors in a community.

• 1. Keystone species are species that have a greater impact on the diversity and species richness in their community than you would predict strictly based on their biomass. For example see Sea stars.
  2. Species richness: the greater the number of species in a community, the greater the number of checks and balances on species numbers, and the less likely it is that the community will experience any change in species richness or composition.